Clearly your actors weren't actually piercing their skin with the needle (otherwise they would be injecting air into their blood stream). So why couldn't there be liquid in the needle?
Shoot it with liquid in it until it's time to inject it and cut to close ups of the plunger being pushed but not having an arm in the shot. Or have the arm in the shot, but have the actual point of contact be hidden behind the arm so you don't see the liquid dripping out.

Realistic liquid simulation is one of the most difficult things to do for any special effects artist. (That and fire. Which is why the latest Harry Potter film's climax in the cave was so amazing.)
I don't know of any good way in AE to fake there being liquid in the syringe for the whole scene. I think a reshoot is your best option.
You could film a syringe with liquid in it and try to match the motion (so that the liquid moves correctly) then stabilize it so you just have the needle sitting still with the liquid moving around. Then motion track that needle onto the needle in the footage. But that's an awful lot of work and I'm not sure it'd look that good.

Now, if you shot your whole scene with liquid in the syringe up to the point of injection, you might be able to fake it. Look at how the liquid looks in the syringe. There shouldn't be bubbles or anything in it when it's actually being injected, so you should be able to just adjust the color of the syringe and mask it as the plunger pushes down...

Also (just for your own knowledge), 8 GB of RAM isn't really that much for AE. My work machine has 16 GB and the new one I just bought for personal use has 24 GB.

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.

There are a few problems with the shot that I didn't mention because I mainly just wanted some general tips on liquids and a direction to head in. Certainly I didn't expect anyone to tell me how to do the shot step by step.

Here's the extra little difficulty - the director wants "magic" drugs in the syringe. I guess it has to glow as well as flow with little sparkly things suspended in the solution. I figured I'd address the flow problem first and then tackle the magic part.
This may not be doable but I didn't know. I will do some camera experiments with a syringe but re-shooting is out of the question, the job wrapped weeks ago.

[Edward Ballinger]"I mainly just wanted some general tips on liquids and a direction to head in... Here's the extra little difficulty - the director wants "magic" drugs in the syringe. I guess it has to glow as well as flow with little sparkly things suspended in the solution. I figured I'd address the flow problem first and then tackle the magic part."

I don't know if you painted yourself into the corner, or if the people who hired you painted you into the corner, but you're in one.

Sorry to have to say this; it's going to sound like a lecture, and it is. The production company for which you shot this is out of its league on the subject of effects work. Somebody didn't do their homework. Someone in the production planning process didn't consider the implications of effects shots. They're not afterthoughts.

But since these shots are indeed afterthoughts are in your case, the best you can do is damage control.

First thing: Michael is absolutely right. Editing will be your friend. The more you can isolate this syringe to a minimum number of shots, the less you'll have to do.

I have a diabetic cat. My wife and I have to give the poor guy insulin shots twice a day. Consequently, I'm pretty familiar with how liquids behave in a syringe. The good news: they don't move much. They don't slosh around. So your problem then becomes one of rotoscoping in an animation of slow-motion particles, and then using blend modes to wrap the exterior of the syringe back around the liquid.

This process won't be fun. It will be very painstaking. It will be very time-consuming. And it will require the services of an excellent rotoscoping and effects practitioner, not someone who admits he "doesn't do much in AE". Don't think I'm out to pick on you. This situation arises frequently enough to warrant a cut & paste response:

Dave's Stock Answer #2:

When you're out on a shoot, and you say, "we'll fix this in post" without knowing PRECISELY HOW you're going to fix it in post, don't shoot it! You'll only end up shooting it over again.

Since post typically costs three times the cost of production, fixing something in post is not a way to save money, but rather a way to spend more of it.

And, before you say "well fix it in post," always consider who's doing the work, especially if you're the one doing the work.

Thanks guys. You've more or less told me what I suspected. Bear in mind that I didn't shoot this thing, I'm late to the game. It's a shoe-string budget that has had two different DPs that shot in short spurts over the course of the last year. I am, in fact, the fall-back editor since the main guy dropped it to go make actual money on a location job overseas.
No, I don't really do AE as a rule. I understand that it's an art and a science but I'll just have to do the best I can.

So the liquid need not move much, that's a relief. Basically I can just motion track the syringe with the "magic potion" and blend a shot of the syringe over it. Sounds like a pain but doable.

Do you have a suggestion about the close ups of the syringe? Any ideas how to make the particles and bubbles in the tube when the plunger is first pulled back?

The director wants it to glow? My guess is that he didn't think about lighting anything so that this "glowing" tube is having any sort of affect on anything around it. Here's a tutorial on lighting up an area near a digital glow.

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.